From my years of listening to music I have noticed that the longer you hear something during a single period of time, without breaks, (especially if the album(s) have words in them that are in a language you know), the more dull the sound becomes.
You eventually tend to zone out and it turns from music into regular volume noise going through your musical device. The length of time it takes for this to happen correlates with how professional the musican is in making their melody and lyrics. And while zoned out, you may zone back in by hearing something very nice. During this zoned out phase you can see how good the artist you are playing is.
My idea for this guide started recently as I was taking a long walk outside, while playing Alizee with headphones on. I had been walking for 2 hours and some minutes, and the second album was halfway through. I had had my ipod on for about an hour and a half, and I was tired of hearing music. I could only focus on the usual things people do while walking. Anyway, the song "Les Collines" was halfway through, and although most people will agree that Alizee is no Britney Spears, the lyrics "Never wanna leave you" came on repeat, and I zoned back into the music, while still paying attention to walking, of course.
So in short my guide to vocals in an album based on all this length of time and zoning out and in would be, make sure your lyrics have something nice in them, on repeat, that everyone would like to hear, somewhere in a song or two in an album. Maybe in the middle of timespan. Before zoning out, I assume memorable lyrics would have the similar effect of keeping you zoned in longer. Melody and rhythm are more complex, but if you listen to music for long periods of time, when you have free time, you have noticed that you can listen to some songs or albums in the same genre for longer periods of time than others
wrote this in 2 mins, sorry for grammar but you get the idea.
Specifically looking for commentary from you two.